1. Field of the Invention
The present invention relates to warm air furnace controls, and more particularly to an electronic control for the forced air blower of a furnace, having an ON-OFF delay circuit in which the ON-time and OFF-time are separately adjustable.
2. Description of the Prior Art
Conventional warm air furnaces measure the temperature in the heat exchanger and turn on the forced air blower when the temperature rises above a selected value. When the temperature drops below a lower selected value, the blower is turned off. Generally, the control is a mechanical thermostat having the hysteresis between the on and off temperatures adjustable. However, improvements in the design of such furnaces to obtain higher efficiencies have involved new materials and reduced wall thicknesses to improve heat transfer. These changes have resulted in a faster drop in heat retained in heat exchangers, allowing higher temperature differentials between the fire side and the air side of the heat exchanger. The maximum temperature and the amount of change in temperature between burner-ON and burner-OFF has been significantly lowered
Existing thermostats depend on the changes in temperature to provide mechanical energy to operate, and these are not suitable to directly control blowers of such modern furnaces directly. One approach to the problem is to utilize a delay-0N timer and a delay-OFF timer connected to an external relay that operates the blower. The timers delay the turn-ON of the blower when the thermostat closes, and lengthens the blower operation when the thermostat opens. To operate the blower when the furnace overheats, a separate control has been provided.
There is a clear need for a low cost, electronic blower control circuit having a single time delay network in which the "ON" delay and "OFF" delay are independently adjustable, with the same circuit monitoring the over-temperature control.